1. Field of the Invention
This invention relates to secured communication, such as links for commercial financial data and company confidential information, and, more particularly, to using a random signal to modulate the amplitude and phase of an RF carrier to enable a low probability of intercept for the communication.
2. Description of Related Art
Conventional secured communications use pseudo-random bit sequences (PRBSs) to encode data. The sequence repeats itself after a finite bit length. The sequence can be detected and synchronized. The encoded data can thus be decoded and extracted by advanced eavesdroppers.
Current low-probability intercept (LPI) communications in airborne, space-borne, and terrestrial vehicles and stations use PRBSs to modulate or scramble transmitting waveforms. The PRBS is not a true random bit sequence. It repeats itself after a fixed bit length. The PRBS is pseudorandom because, unlike truly random sequences, it is deterministic and after a certain number of bits or elements the sequence starts to repeat itself. Truly random sequences, such as sequences generated by radioactive decay or by white noise, do not have a period and do not repeat.
FIG. 1 is a schematic diagram of a PRBS system. A PRBS generator 10 is used to generate a PRBS 20. The PRBS generator 10 receives power from power supply 15 for its various internal elements, such as transistors, and a control signal 17, such as a transistor gate voltage, to begin the generation of the pseudorandom sequence. The PRBS 20 repeats itself after a period 25 that is denoted as T. Because the PRBS 20 is not a truly random bit sequence it may be decoded. The PRBS modulated LPI waveforms are thus susceptible to intercept by an eavesdropper equipped with advanced receivers and analysis algorithms.
Therefore, there is a need for a truly secured communication via optical fiber or wireless that prevents the eavesdroppers from decoding and extracting the data being communicated.